Many wireless communication systems utilize a downlink (DL) transmission scheme based on an orthogonal frequency division multiple access (OFDMA) air interface. One example of such a communication system is the 3GPP long term evolution (LTE) system. In LTE, for example, wireless transmit/receive units (WTRUs) receive their data (and in some cases their control information) on the physical downlink shared channel (PDSCH). The transmission of the PDSCH is scheduled and controlled by an evolved Node-B (eNB) using the so-called downlink scheduling assignment, which is carried on a physical downlink control channel (PDCCH). As part of the downlink scheduling assignment, the WTRU receives control information on the modulation and coding set (MCS), and a downlink resource allocation (i.e., the indices of allocated resource blocks). The WTRU receives its PDSCH on allocated downlink resources with corresponding MCS.
In order to further improve achievable throughput and coverage of OFDMA-based radio access systems, carrier aggregation may be used. In the LTE advance (LTE-A) system, for example, carrier aggregation is supported and the PDSCH can be transmitted on more than one assigned carrier. Accordingly, PDCCHs should be able to carry the control information for the carriers that are assigned for PDSCH transmission. If there are M carriers, there should be M sets of control information for PDSCH transmission. Each set of control information can be carried by one PDCCH, which is encoded separately from other PDCCH. In this case, there are M PDCCHs for M carriers. The encoded PDCCHs can be transmitted on one or multiple carriers. Alternatively, M set of control information can be encoded jointly and carried by one single joint PDCCH. In this case, there is only one PDCCH but with a larger size for downlink control information (DCI) format. The joint PDCCH can then be transmitted in one or multiple carriers.
In LTE-A, several carriers can be aggregated. There are two possible schemes to encode the PDCCH, namely separate coding and joint coding of the PDCCH. In separate coding of the PDCCH, each PDCCH is encoded separately from one another regardless of the number of carriers being aggregated. It is simple, straightforward and backward compatible. However, the scheme has very high blind decoding complexity, which increases power consumption linearly with number of carriers being aggregated, particularly if PDCCHs are transmitted in all carriers. Schemes proposed for the downlink control channel in LTE-A must be backward compatible.
In joint coding, several PDCCHs are jointly encoded. By doing so, some overhead can be reduced, such as cyclic redundancy check (CRC) bits. However, joint coding of PDCCHs may require several DCI formats of different sizes, which increases the blind decoding complexity. For example, if dynamic DCI format is used, depending on the number of assigned carriers, different DCI formats should be used. One DCI format may be used for one carrier that is assigned; a different DCI format is used for two carriers that are assigned. Another DCI format is used for three carriers that are assigned, and so on. As a result, M DCI formats of different lengths are required for M carriers that are configured.